4-t-Butylcyclohexanol esters, in particular, 4-t-butylcyclohexyl acetate, are publicly known perfume ingredients.
4-t-Butylcyclohexyl acetate has cis- and trans-stereoisomers with respect to the t-butyl and acetoxy groups bonded to the cyclohexane ring. Many of products marketed today are in the form of mixtures of these isomers containing about 30 to 70% by weight of the cis-isomer. However, it is known that these cis- and trans-isomers largely differ from each other in their fragrance qualities and the cis-isomer is preferable to the trans-isomer S. Arctander, Perfume and Flavor Chemicals, Montclair, N.J. (1969), monograph No. 441!. Accordingly, 4-t-butylcyclohexyl acetate with an elevated content of the cis-isomer has been required in the perfume industry.
4-t-Butylcyclohexyl acetate is obtained by acetylation of 4-t-butylcyclohexanol. The conventional method for producing this 4-t-butylcyclohexanol employed as the starting material comprises, for example, hydrogenating 4-t-butylphenol at 160.degree. C. by using Raney nickel as a catalyst Beilstein Handbook of Organic Chemistry, E III 6, p. 126 (1960)). However, the 4-t-butylcyclohexanol obtained by this method contains the cis-isomer only in a small amount of about 20 to 30% by weight. Therefore, the 4-t-butylcylohexyl acetate obtained by acetylating this product contains only about 20 to 30% by weight of the cis-isomer, which makes it undesirable as a perfume ingredient.
Accordingly, there have been proposed several methods for producing 4-t-butylcyclohexanol containing a large amount of the cis-isomer by stereoselectively hydrogenating 4-t-butylcyclohexanone with the use of various metal catalysts, as will be shown hereinbelow.
(a) A method wherein 4-t-butylcyclohexanone is hydrogenated by using an iridium-phosphinic acid complex in isopropanol E. L. Ernest et al., Organic Synthesis, vol. 50, pp. 13-15 (1970)!. PA1 (b) A method wherein 4-t-butylcyclohexanone is hydrogenated by using a rhodium catalyst in the presence of conc. hydrochloric acid in isopropanol or tetrahydrofuran Shigeo Nishimura et al., Chemistry Letters, pp. 963-966 (1977)!. PA1 (c) A method wherein 4-t-butylcyclohexanone is hydrogenated by using a catalyst system composed of a combination of rhodium with tetrahydroborate, etc., on a specific carrier (JP-B-7-64769; the term "JP-B" as used herein means an "examined Japanese patent publication"). PA1 wherein X.sup.1 represents a halogen atom or a group represented by "R.sup.1 COO", wherein R.sup.1 represents a hydrogen atom, an alkyl group having from 1 to 4 carbon atoms or a halogenated alkyl group having from 1 to 4 carbon atoms; L represents an organic phosphine compound; a and b are each an integer of from 0 to 2, provided that (a+b) is 2; when L is monodentate ligand, c is an integer of from 3 to 4; and when L is bidentate ligand, c is an integer of from 1 to 2; PA1 wherein R.sup.2, R.sup.3 and R.sup.4 may be the same or different and each represents an optionally substituted alkyl group, an optionally substituted aralkyl group or an optionally substituted aryl group; or a bidentate ligand represented by the following formula (V): EQU R.sup.5 R.sup.6 P--A.sup.1 --PR.sup.7 R.sup.8 (V) PA1 wherein R.sup.5, R.sup.6, R.sup.7 and R.sup.8 may be the same or different and each represents an optionally substituted alkyl group, an optionally substituted aralkyl group or an optionally substituted aryl group; and A.sup.1 represents an optionally substituted alkylene group, --A.sup.2 --Ar--Ar--A.sup.2 -- or --Ar--Ar--, wherein A.sup.2 represents an optionally substituted alkylene group, and --Ar--Ar-- represents a 1,1'-biphenyl group having a binding arm at the 2,2'-position, a 1,1'-binaphthyl group having a binding arm at the 2,2'-position, or a 5,5',6,6',7,7',8,8'-octahydro-1,1'-binaphthyl group having a binding arm at the 2,2'-position, wherein the biphenyl group may be substituted by a methyl, methoxy or dialkyl-substituted amino group and the binaphthyl group may be substituted by an alkali sulfonate. PA1 wherein X.sup.1 represents a halogen atom or a group represented by "R.sup.1 COO", wherein R.sup.1 represents a hydrogen atom, an alkyl group having from 1 to 4 carbon atoms or a halogenated alkyl group having from 1 to 4 carbon atoms; L represents an organic phosphine compound; a and b are each an integer of from 0 to 2, provided that (a+b) is 2; when L is monodentate ligand, c is an integer of from 3 to 4; and when L is bidentate ligand, c is an integer of from 1 to 2. PA1 RUH.sub.2 (PPh.sub.3).sub.4, PA1 RuHCl(PPh.sub.3) .sub.4, PA1 RuH(HCOO) (PPh.sub.3).sub.3, PA1 RuH(CH.sub.3 COO) (PPh.sub.3).sub.3, PA1 RuCl.sub.2 (PPh.sub.3).sub.3, PA1 RuCl.sub.2 (PPh.sub.3).sub.4, PA1 RuBr.sub.2 (PPh.sub.3) .sub.4, PA1 RuI.sub.2 (PPh.sub.3).sub.4, PA1 RuCl.sub.2 P(CH3)Ph.sub.2 !.sub.4, PA1 RuCl.sub.2 P(CH.sub.3).sub.2 Ph!.sub.4, PA1 RuCl.sub.2 P(CH.sub.3).sub.3 !.sub.4, PA1 RuCl.sub.2 Ph.sub.2 P--(CH.sub.2).sub.2 --PPh.sub.2 !.sub.2, PA1 RuCl.sub.2 (CHIRAPHOS).sub.2, PA1 RuCl.sub.2 (BINAP), PA1 Ru(CH.sub.3 COO).sub.2 (Tol-BINAP) and PA1 Ru(CF.sub.3 COO).sub.2 (Tol-BINAP). EQU Complex 2: (RuH.sub.d L.sub.e)(X.sup.2).sub.f (VI) PA1 wherein X.sup.2 represents ClO.sub.4, PF.sub.6 or BF.sub.4 ; L is as defined above; when L is monodentate ligand, e is 2 and f is 2 when d is 0; and e is 4 and f is 1 when d is 1; and when L is bidentate ligand, e is 1 and f is 2 when d is 0; and e is 2 and f is 1 when d is 1. PA1 Ru(BINAP)!(ClO.sub.4).sub.2, PA1 Ru(m-Tol-BINAP)! (PF.sub.6).sub.2, PA1 Ru(MeO-BINAP)! (BF.sub.4).sub.2, PA1 RuH(BIPHEMP).sub.2 !C10.sub.4 and PA1 RuH(t-Bu-BINAP).sub.2 ! PF.sub.6. EQU Complex 3: (RuX.sup.3)(Bz)L.sub.h !(X.sup.4).sub.g (VII) PA1 wherein X.sup.3 represents a halogen atom; Bz represents optionally substituted benzene; X.sup.4 represents a halogen atom, ClO.sub.4, PF.sub.6, BF.sub.4 or BPh.sub.4, wherein Ph represents a phenyl group, the same will apply hereinafter; L is as defined above; when L is monodentate ligand, h is 2 and g is 1 or g may be 3 when X.sup.3 and X.sup.4 are each an iodine atom; and when L is bidentate ligand, h is 1 and g is 1 or g may be 3 when X.sup.3 and X.sup.4 are each an iodine atom. PA1 RuCl(benzene)(BINAP)!Cl, PA1 RuI(benzene) (Tol-BINAP)!I, PA1 RuI(p-cymene)(Tol-BINAP)!I and PA1 RuI(p-cymene)(BINAP)!I.sub.3. EQU Complex 4: (Ru.sub.2 Cl.sub.4 L.sub.w)(T) (VIII) PA1 wherein T represents a tertiary amine; and L is as defined above; when L is monodentate ligand, w is 4; and when L is bidentate ligand, w is 2. PA1 Ru.sub.2 Cl.sub.4 (BINAP).sub.2 !(NEt.sub.3) and PA1 Ru.sub.2 Cl4(DM-BINAP).sub.2 !(NEt.sub.3). PA1 wherein M represents an alkali metal or an alkaline earth metal; and R.sup.9 represents a hydroxyl group, an alkoxy group having from 1 to 4 carbon atoms or a mercapto group. Preferable examples of the base include KOH, Ca(OH).sub.2, KOCH.sub.3, KOC(CH.sub.3).sub.3, LiOH, LiOCH.sub.3, LiOC(CH.sub.3).sub.3 and NaOH. Among all, those containing alkali metals are preferable and KOH and NaOH are particularly preferable therefor. PA1 wherein R.sup.10, R.sup.11 and R.sup.12 are the same or different and each represents a hydrogen atom, an optionally substituted alkyl group, an optionally substituted aralkyl group or an optionally substituted aryl group, provided that R.sup.10, R.sup.11 and R.sup.12 do not represent hydrogen atoms at the same time; primary, secondary or tertiary diamines represented by the following general formula (XI): EQU NR.sup.13 R.sup.14 --Z--NR.sup.15 R.sup.16 (XI) PA1 wherein R.sup.13, R.sup.14, R.sup.15 and R.sup.16 are the same or different and each represents a hydrogen atom, an optionally substituted alkyl group, an optionally substituted aralkyl group or an optionally substituted aryl group; and Z represents an optionally substituted, saturated or unsaturated carbon chain having from 1 to 6 carbon atoms or an optionally substituted, saturated or unsaturated carbon ring having from 3 to 6 carbon atoms; and other cyclic amines. PA1 instrument: HP-5890 (manufactured by Hewlett-Packard, Co.) PA1 column: HP-20M fused silica capillary column (0.20 mm.times.25 m) (manufactured by Hewlett-Packard, Co.) PA1 measuring temperature: 55.degree.-215.degree. C. (program rate 4.degree. C./min) PA1 injection temperature: 250.degree. C. PA1 carrier gas: helium (0.6 ml/min) PA1 instrument: M-2000A (manufactured by Hitachi, Ltd.) and HP-5890 Series II (manufactured by Hewlett-Packard, Co.) PA1 column: BC-WAX (0.25 mm.times.50 m, 0.15 .mu.m) (manufactured by GL Sciences Inc.) PA1 instrument: IR-810 (manufactured by JASCO Corporation) PA1 instrument: Model AMX-400 (400 MHz), FT-NMR analyzer (manufactured by Bruker JAPAN Co., LTD.) PA1 internal standard: tetramethylsilane. PA1 instrument: Model AMX-400 (100 MHz) (manufactured by Bruker JAPAN Co., LTD.).
In addition, there has been proposed a method wherein 4-t-butylcyclohexanone is hydrogenated by using lithium trisamylborohydride without using any catalytic reaction H. C. Brown et al., J. Am. Chem. Soc., vol. 98, pp. 3383-3384 (1979)!.
By using these methods, 4-t-butylcyclohexanol containing the cis-isomer at a high ratio of 80% by weight or above can be obtained. To perform on the industrial scale, however, each of these methods suffers from economical problems. Namely, an expensive catalyst should be used in a large amount in each method. In the method (c), for example, a catalyst containing rhodium, which is a very expensive material, should be used at a molar ratio to the starting 4-t-butylcyclohexanone (i.e., the substrate) of 1/20 or more molar ratio of substrate/catalyst.ltoreq.20!. In the method (a), it is necessary to employ trimethyl phosphite having an intensely offensive odor and a strong acid in large amounts, which elevates the costs of the equipments for ventilation and drainage.
As discussed above, there has been established no industrially available process for producing 4-t-butylcyclohexanol with a large cis-isomer content. Under the existing circumstances, therefore, 4-t-butylcyclohexyl acetate products in the form of mixtures of the cis-isomer with the trans-isomer at a weight ratio of about 30 to 70, which are less expensive but inferior in the value as a perfume, are still manufactured and sold on a large scale, though there are marketed only in a small amount expensive 4-t-butylcyclohexyl acetate products rich in the cis-isomer.
Accordingly, an object of the present invention is to provide a process for economically producing 4-t-butylcyclohexanol having a large content of the cis-isomer, which is highly valuable as a perfume ingredient, at a low cost on an industrially available scale.
Under these circumstances, the present inventors have conducted extensive studies to achieve the above-mentioned object. As a result, they have successfully found that cis-4-t-butylcyclohexanol having a high purity can be obtained by stereoselectively hydrogenating 4-t-butylcyclohexanone by using a catalyst containing inexpensive ruthenium in the presence of a base having an alkali metal or an alkaline earth metal and an amine, thus completing the present invention.